Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
1999-07-06
2002-10-08
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S087000, C345S063000, C345S089000, C348S674000, C382S274000, C382S167000
Reexamination Certificate
active
06462735
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a display device having an electro-optical device such as a liquid crystal device, or the like, in which transmittance (reflectance) varies with applied voltage.
2. Description of Related Art
Electronic equipment incorporating such a display device includes, for example, a projection display device in which three liquid crystal devices (LCD) for performing optical modulation of each chromatic light of red (R), green (G), and blue (B) are respectively used as a light valve (L/V).
FIG. 16
shows applied voltage-transmittance (V-T) characteristics inherent in each of the liquid crystal devices for performing optical modulation of each chromatic light of R, G, and B (in which twisted nematic liquid crystal is used). In
FIG. 16
, although the range for applying voltage to the liquid crystal can be set between 0V and 6V, both the white level region in which transmittance is approximately 100% and the black level region in which transmittance is approximately 0% are saturated. Accordingly, such an arrangement is provided so that amplitude of the voltage for application to the liquid crystal is limited to a level of approximately 3.8V so as to prevent the white level region and the black level region from becoming saturated. That is, in this arrangement, direct current bias (DC bias) of a picture signal is adjusted in such a manner that voltage which does not produce saturation in the regions of the white level and black level is applied to each liquid crystal. This is referred to as brightness adjustment.
In
FIG. 17
, the V-T characteristics of the chromatic lights R, G, and B present a mutually different inclination, and there is shown variations in the voltage which produces saturation in the regions of the white level and the black level among the colors. In order to reduce the variations, a gain of a picture signal is adjusted, which is referred to as gain adjustment. Furthermore, in the liquid crystal device (LCD), transmittance is determined by the ratio of light amounts which are transmitted through a liquid crystal panel having liquid crystal between a pair of substrates and through a polarizer disposed on at least one side of the liquid crystal panel. Reflectance is substituted for transmittance in a reflection-type electro-optic device.
FIG. 17
shows a relationship of a gray-scale value and transmittance of a digital input signal after brightness adjustment and gain adjustment. As shown in
FIG. 17
, in the black level region, there is a slight change in transmittance with respect to changes of gray-scale values, so that satisfactory resolution cannot be obtained.
In order to obtain ideal gamma characteristics (ideal &ggr; characteristics) as shown in
FIG. 17
, gamma correction characteristics shown in
FIG. 18
are used for correcting a digital picture signal. When the liquid crystal device is driven based on the signal subjected to gamma correction performed according to the characteristics shown in
FIG. 18
, characteristics close to the ideal gamma characteristics in
FIG. 17
can be obtained as shown in FIG.
19
. Thus, conventionally, in order to obtain gamma correction characteristics, it is essential to perform brightness adjustment and gain adjustment in advance.
Consequently, a display device incorporating a conventional liquid crystal device requires a circuit arrangement as shown in FIG.
15
. In
FIG. 15
, a picture signal is converted into a digital picture signal by an analog-to-digital (A/D) converter
10
to perform signal-processing including gamma correction and digital-to-analog (D/A) conversion by a picture signal processing circuit
20
. The picture signal processing circuit
20
includes ASIC
22
having a gamma correction circuit and a D/A converter
24
. An amplifier
30
performs gain adjustment of the analog picture signal, and then a bias adjustment circuit
40
performs DC bias adjustment (brightness adjustment) of the signal so as to send it to a liquid crystal device
50
.
A CPU
60
shown in
FIG. 15
serves as a controller of the display device. The CPU
60
controls gamma correction as follows. First, the V-T characteristics which are inherent in the liquid crystal device
50
shown in
FIG. 16
are actually measured. Next, the CPU
60
controls gain adjustment performed by the amplifier
30
through a gain controller
80
, and also controls DC bias performed by the bias adjustment circuit
40
through a brightness controller
90
to obtain the V-T characteristics shown in FIG.
17
. An EEPROM
70
stores the obtained the V-T characteristics, the gain adjustment data, and the brightness adjustment data, from which the V-T characteristics are obtained. Then, the CPU
60
calculates gamma correction characteristics shown in
FIG. 18
, based on the V-T characteristics stored in the EEPROM
70
, which is shown in
FIG. 17
, and predetermined ideal gamma characteristics, and sets the gamma correction characteristics, for example, as table information, in a gamma correction circuit inside the ASIC
22
. Accordingly, in order to obtain the V-T characteristics as shown in
FIG. 19
in the liquid crystal device
50
, such an arrangement is provided so that a picture signal input from the A/D converter
10
is corrected according to table information so as to perform gain adjustment and brightness adjustment by the amplifier
30
and the bias adjustment circuit
40
according to the gain adjustment data and the brightness adjustment data stored in the EEPROM
70
.
Therefore, it is essential for the conventional display device described above to perform such gain adjustment and brightness adjustment in advance before determining gamma correction characteristics, as shown in
FIG. 18
, in the gamma correction circuit inside the ASIC
22
. Such gain adjustment and brightness adjustments are extremely complicated since they vary among liquid crystal devices. In addition, precise adjustment is necessary, since misadjustment directly affects image quality. More specifically, since a projection display device which synthesizes chromatic lights modulated by a plurality of liquid crystal devices for projection requires mutual adjustment between the liquid crystal devices, performing gamma correction requires very complicated work.
In addition, since the conventional display device performs brightness adjustment, the transmittance range of the liquid crystal device is narrowed, leading to reduction in contrast and darkening of the display screen. In other words, the conventional brightness adjustment and gain adjustment result in the transmittance, for example, of 3% for the black level and of 97% for the white level in the liquid crystal device. This allows contrast reduction and increased darkening of the screen compared with a device which uses the whole transmittance range of 0 to 100%, as described above.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a display device and electronic equipment which can yield the brightness and contrast of an electro-optical device as close to the best characteristics that the device can offer.
It is another object of the present invention to provide a display device and electronic equipment in which gain adjustment and brightness adjustment are not necessary.
It is another object of the present invention to provide a display device and electronic equipment which can reproduce ideal input-output characteristics in terms of gamma correction and color temperature.
According to the present invention, there is provided a display device including an electro-optical device in which transmittance changes according to voltage applied to an electro-optical material, a digital gamma correction circuit for performing gamma correction of a digital picture signal, a digital-to-analog conversion circuit for converting the digital picture signal corrected by the digital gamma correction circuit into an analog picture signal, and an amplifier for amplifying the analog picture signal, in which voltage is appli
Hjerpe Richard
Nguyen Francis
Seiko Epson Corporation
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